Pressure sensors generally contain movable or deformable bodies, most often a deflectable diaphragm, and they can be either of the main types absolute pressure sensors and differential or relative pressure sensors. An absolute pressure sensor measures a pressure in relation to a vacuum pressure, the latter one generally existing in a relatively small cavity located at one surface of the diaphragm, the pressure to be measured acting on the opposite surface. A differential pressure sensor measures the difference of two pressures acting on opposite surfaces of the diaphragm. Some intermediate special types include gauges and sealed gauges where the pressure to be measured is measured in relation to a reference pressure, the reference pressure for sealed gauges existing in a closed cavity located directly at a surface of the diaphragm, see Gregory T. A. Kovacs, “Micromachined transducers handbook”, WCB/McGraw-Hill New York 1998, pp. 248–258. The movement or deformation of the diaphragm can be sensed in different ways such by measuring the change of the capacitance of a suitable adapted capacitor, measuring the change of electric characteristics of a piezoresistive body or the change of the resistance of an electrical conductor coupled to the movement of the diaphragm and thereby being in varying strained states. For micromachined pressure sensors two major manufacturing methods are employed, bulk micromachining and surface micromachining, see e.g. the cited book by Kovacs and the article by Martin A. Schmidt, “Silicon wafer bonding for micromechanical devices”, Solid State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 13–16, 1994, pp. 127–131.
Absolute pressure sensors and sealed gauge pressure sensors both need a hermetic sealing of a relatively small cavity at the active diaphragm to get a reference pressure, preferably a vacuum enclosure. This can be accomplished on a wafer basis using e.g. silicon wafer bonding under vacuum conditions. The two dominant bonding techniques are silicon direct bonding, also called silicon fusion bonding, see e.g. the article by Schmidt cited above, S. Mack, H. Baumann, U. Gösele, “Gas development at the interface of directly bonded silicon wafers: investigation on silicon-based pressure sensors”, Sensors and Actuators A, Vol. 56, 1996, pp. 273–277, C. Harendt, B. Höfflinger, H. G. Graf and E. Penteker, “Silicon direct bonding for sensor applications: Characterization of the bond quality”, Sensors and Actuators A, Vol. 25–27, 1991, pp. 87–92, and anodic bonding, see e.g. H. Henmi, S. Shoji, K. Yoshimi and M. Esahi, “Vacuum packaging for microsensors by glass-silicon anodic bonding”, Sensors and Actuators A, Vol. 43, 1994, pp. 243–248. Other possible techniques to achieve vacuum sealing of microcavities are by metal evaporation, see M. Bartek, J. A. Foerster, R. F. Wolfenbuttel, “Vacuum sealing of microcavities using metal evaporation, Sensors and Actuators A, Vol. 61, 1997, pp. 364–368, and by sealing using LPCVD, see Carlos H. Mastrangelo, James Hsi-Jen Yeh and Richard S. Muller, “Electrical and optical characteristics of vacuum-sealed polysilicon lamps”, IEEE Trans. on Electron Devices, Vol. 39, No. 6, June 1992, pp. 1363–1375, and S. Sugiyama, T. Suzuki, K. Kawabata, K. Shimaoka, M. Takigawa et al., “Microdiaphragm pressure sensor”, IEDM Tech. Dig., 1997, pp. 184–187.
Generally, for example for use in an in vivo application such as a sensor attached to a catheter, a small micromechanical piezoresistive absolute pressure sensor is desired, having a high pressure sensitivity and a controlled temperature behaviour and a high long term stability. It should not be affected or change performance due to changes in the environment, e.g. it should not be affected by a humid environment. Also a designing and manufacturing process is desired, which is suitable for volume production, i.e. a batch fabrication process, with a high yield using standard micromachining process steps on a wafer level. A higher strain gauge factor, in comparison to pressure sensors existing today, is also desired.
It can be expected that micromachined devices having their essential parts made from monocrystalline material will have good long-term characteristics. Thus, pressure sensors having bonded monocrystalline diaphragms have been proposed. However, the handling of the very thin and therefore delicate monocrystalline diaphragms is very costly and can hardly be used in a process for mass fabrication. There are also problems as to pressure in the reference chamber when using bonding methods, e.g. direct or fusion bonding.
A problem associated with vacuum sealing using direct fusion bonding is that even if the bonding is performed under a reduced pressure the residual gas pressure inside the reference cavity after bonding is considerably higher than the original chamber pressure, which in turn gives problem with the temperature sensitivity. Vacuum sealing under UHV (Ultra High Vacuum) conditions is difficult and not suitable for production.
Another problem associated with direct fusion bonding includes voids between the bonded surfaces, due to problems with for example particles on the wafer surfaces before bonding. This significantly decreases the process yield.
A problem associated with surface micromachining and the use of polycrystalline silicon is that the material properties are not optimized, such as the gauge factor in the strain gauges, the diffusion through the diaphragm etc.
Different material in the same structure gives different temperature expansion coefficients.
A long process time including many steps is generally required.
The strain gauge has to be protected since it is placed on top of the diaphragm.
A better performance would probably be achieved with a planar and not so rough surface, which is the case in surface micromachining.
Pressure sensors based on SOI-substrates have been proposed, see U.S. Pat. Nos. 6,131,466 for Vigna et al., U.S. Pat. No. 5,510,276 for Diem et al. and U.S. Pat. No. 5,095,401 for Zavracky et al. Also, in U.S. Pat. No. 5,335,550 for Satou a method of producing a semiconductor pressure sensor having a monocrystalline diaphragm is disclosed. A lower silicon substrate having a recess is bonded to an upper silicon substrate having an oxide layer on its bottom surface. Thereafter, part of the upper substrate is removed by e.g. machining to produce a diaphragm. The accuracy of the thickness obtained of the diaphragm is dependent on the kind of process used for removing part of the upper substrate. Using machining as disclosed in this patent, the accuracy will not be very high. Also, the fact that a large amount or height of material must be removed will give a thickness of the diaphragm that cannot be very accurately defined and that can also have thickness variations.